Transferring Technology from Conservation Science to Infrastructure Renewal
In the context of highway research, technology transfer usually refers to the adaptation of software or hardware from the aerospace or defense fields to problems of highway construction and maintenance. These fields are not, however, the only sources of relevant technology. A surprising amount of technology developed for the conservation of art and architecture can be directly transferred to the renewal of the transportation infrastructure.
In the United States, "conservation" refers to technical measures taken to protect cultural property from the destructive forces of nature. (This concept should not be confused with historic preservation, which has come to mean the legal and financial measures taken to preserve cultural property from being torn down to make way for new construction.) Architectural conservation involves prolonging the lives of structures that stand outdoors. Objects conservation refers to the conservation of works of art, such as oil paintings, that are kept indoors (e.g., museums). The techniques and methods of the two fields of conservation science can differ considerably with the size of the object being conserved, the working conditions, and the environment.
Conservation science involves the use of scientific tools and methods to: (1) understand the causes of deterioration, (2) inspect objects or monuments nondestructively, and (3) prescribe the most suitable methods for treatment or restoration. These three functions match the high-priority areas defined in a recent National Science Foundation report on strategic issues in civil infrastructure systems research. (1) In that report, these functions are called deterioration science, assessment technologies, and renewal engineering, respectively.
Not surprisingly, some of the most valuable achievements of conservation science are rooted in deterioration science, which studies the long-term behavior of materials in the environment. Conservation scientists, by definition, examine objects and structures that have been in existence for many centuries. Their perspective provides unique insight into how materials deteriorate over time.
Metal corrosion is an area of study to which conservation scientists have made significant contributions. Conservation science is concerned with metal corrosion as it relates both to protection and to aesthetics. From a protection standpoint, the formation of a stable corrosion layer on a metal surface greatly slows down the rate of loss of the metal. If, however, the corrosion layer is unstable--like rust on iron or steel--it flakes off, exposing more bare metal and allowing the rate of corrosion to accelerate. Concerning aesthetics, artists and sculptors have often deliberately treated metal surfaces with mixtures of chemicals to produce a specific kind of corrosion layer, a patina, that is intended to enhance the beauty of the object. Conservation scientists may thus be called upon not only to protect the surface of a corroded metal object but also to restore the original patina created by the artist.
A major contribution of conservation science has been to recognize that these aspects of corrosion layers should be treated as problems in applied mineralogy rather than as the oxidation of bare metals. (2) This approach strongly contrasts with that of industrial corrosion engineers, who are primarily concerned with the prevention or control of oxidation. In the long run, however, metals will inevitably tend to oxidize, particularly when exposed to the atmosphere. Once this happens, the metal oxides or hydroxides then react with chemicals in the environment to produce complicated layers of minerals containing sulfates, chlorides, carbonates, etc. These complex corrosion layers are the ones typically investigated by conservation scientists. They use physical chemical models to help them predict the stability of the minerals under possible exposure conditions.
The case of the Statue of Liberty's skin illustrates this approach. During the major restoration work performed on the statue in 1986, some scientists noticed that the copper skin of the surface showed a distinctive pattern of light or dark blue patches. They theorized that this pattern was caused by acid rain attacking the metal, since the oxidation of copper by such acids as sulfuric acid is a well-known phenomenon.
After 100 years, the surface the statue presents to the environment is no longer bare metal, but a coating of minerals. The issue, then, did not involve copper, but rather how this combination of minerals would respond to acid rain. A physical chemical model was developed to predict the stability of the various complicated copper minerals known to exist in the patina and their solubility in thin layers of acidic water. (3)
This model required more than 10 equations. To make the results of such models more comprehensible, they are typically graphed in a phase diagram. Phase diagrams represent an essential tool in materials science. By selecting an appropriate pair of axes, a multidimensional mathematical surface can be presented on a two-dimensional plane to allow for the best possible visualization of a specific problem. (See figure 1.) The Statue of Liberty phase diagram showed that the statue's patina would not be damaged by acid rain--at least not at the levels found in the rain falling on New York City. The phase diagram also revealed that the color of the statue's surface at any point was determined by the local microclimate of the relative exposure to sea spray, rainwater, and sulfur dioxide air pollution.
The kind of phase diagram developed for the Statue of Liberty is now being applied to the very urgent problem of the corrosion of steel reinforcements in concrete highway structures. The conversion of the steel to rust creates expansive forces that lead to the cracking and spalling of the overlying concrete. This phenomenon is a major cause of damage to the transportation infrastructure. The chemical model for this problem is much more complicated than that for the Statue of Liberty, involving more than 25 equations. Moreover, this model cannot be reduced to a single phase diagram but will require several such diagrams. Alternatively, the results may be presented via computer to produce a virtual reality display. This approach replaces the two-dimensional phase diagram with a computer-generated multidimensional virtual space. By traveling vicariously along specified paths in this space, the materials scientist can readily visualize the effects of different conditions on the corrosion process.
Another problem in concrete deterioration can be approached using a phase diagram developed for the weathering of medieval stained glass. (4) Many of these windows in cathedrals in Germany and other countries have developed crusts and pitted areas. (See figures 2 and 3.) This kind of damage is related to the composition of the glass itself. Stained glass that contains a high ratio of potassium to sodium and calcium is prone to react with atmospheric moisture, producing a gel-like material that hardens over time into a crust. This process is shown graphically in figure 4, which is another type of phase diagram. In this triaxial diagram, regions of unreactive and reactive combinations of potassium, sodium, and calcium are indicated.
This phase diagram is relevant to the problem of alkali-silica reactivity (ASR) in concrete. Certain types of silica stone aggregate in concrete containing relatively high concentrations of the alkali elements--i.e., potassium and sodium--will produce a gel. (5) This gel is very expansive and can eventually crack open the concrete; this can occur so gradually that the damage may not be observed for decades. The reactions creating the gel are not yet completely understood. The chemical constituents involved, however, are the same as those in the weathering of stained glass. Therefore, lessons learned in the conservation of these windows could be applicable to solving the ASR problem.
Conservation science can provide information on other aspects of the durability of concrete over long periods of time. These long-term considerations are particularly important for the construction of nuclear waste storage facilities that must have service lives of tens of thousands of years.
Although we think of concrete as a quintessentially modern material, many structures were, in fact, built of concrete during the Roman and Byzantine periods; some of these, such as Hagia Sophia in Istanbul, are still standing. (6) Materials scientists are studying samples of ancient concrete from Hagia Sophia and Hadrian's Wall in Great Britain (among other structures) to determine long-term changes in the microstructure of concrete. One such change is carbonation--a reaction in which carbon dioxide from the atmosphere penetrates the concrete and converts it to carbonate compounds. (7) Carbonation can either improve the strength of concrete or promote its degradation.
Strictly speaking, ancient concrete did not use the same starting materials as does modern portland cement concrete, which was invented in the 19th century. For example, the most important constituent of portland cement concrete is alite, a highly reactive calcium trisilicate compound. Also, the cement's manufacture requires high-temperature industrial technology. In contrast, the concrete used in antiquity was produced from a mixture of slaked lime and a pozzolan. This pozzolan was often crushed brick although sometimes when impure limestone was calcined to make the lime, a calcium disilicate compound called belite was inadvertently made. Belite is another, less reactive constituent of portland cement concrete.
Despite the differences in the composition of the starting materials, many of the reaction products generated in the hardening process are the same for ancient concrete and for portland cement concrete. In addition, ancient concrete changes through the same processes, such as carbonation, as does modern concrete.
The study of ancient material can provide clues on how to make better concrete, as well as to explain how concrete deteriorates. For example, one approach for fighting the ASR problem is to add pozzolans. Modern pozzolans are often waste products from other industries such as silica fume from ferrosilicon refineries or slag from steel blast furnaces. Fly ash from coal-fired electric power plants is also used; this fly ash has the same composition as the ground-up brick used to make the concrete.
Not all pozzolans are manufactured. The name itself is taken from the town of Puteoli, near Naples, where the local volcanic rock produced superior results when added to mortar. Other natural pozzolans used in antiquity came from sources in Turkey and the Greek Aegean island of Santorini. (8) Researchers are investigating these deposits for use in modern concrete. (9)
In addition to concrete and metals, a version of asphalt was also used in antiquity. Asphalt pavements have been discovered in archaeological excavations of ancient Babylonian cities. (10) This asphalt was not manufactured; modern asphalt is a byproduct of petroleum refining. But the ancient asphalt is a natural equivalent--the residue left behind when crude oil seeps out of the ground and the more volatile fractions evaporate.
Sometimes ancient knowledge is relearned painfully. For example, when the ancient Greeks built the Parthenon, they reinforced the marble structure with iron dowels and cramps. As in modern reinforced concrete, iron would rust and expand, cracking the marble. The ancient Greeks were aware of this problem, and to prevent it, they sheathed the iron with lead.
At the start of the 20th century, the Parthenon, which had been blown up in the 16th century in a war between Venice and the Ottoman Empire, was reconstructed. However, iron dowels without lead were used. Rusting soon began, and serious spalling of the marble occurred. Another restoration campaign was launched in the 1970s. This time, each of the iron reinforcements was located in the stone by gamma-ray radiography. The structure was then dissembled and the iron was removed. In the iron's place, titanium fittings were installed. No further corrosion damage is anticipated for the Parthenon.
The second function identified by the National Science Foundation as a high-priority area in civil infrastructure systems research is assessment techniques. In this area, conservation scientists have made intensive use of advanced analytical instruments and methods to characterize art and architectural materials and their deterioration products. The high value placed on these objects has justified research into more advanced analytical methods. In the laboratory, techniques such as x-ray diffraction and microchemistry have been used to study corrosion layers. These techniques are also found in some conventional civil engineering materials-testing laboratories. The challenges of conserving valuable artifacts have stimulated the use of unconventional methods as well; these include Raman spectroscopy and neutron diffraction. A neutron diffraction pattern of the mortar from Hagia Sophia identified some pozzolanic reaction products. Incidentally, the presence of CACH (tetracalcium aluminate carbonate) is a sign that the original cementitious minerals have carbonated over the centuries.
Nondestructive testing has been a major area of emphasis in conservation science because of the need to avoid destroying cultural materials. Several methods have been employed in this nondestructive testing, including infrared thermography, ground-penetrating radar, and ultrasonics. These methods are used to look into walls and foundations to locate voids as well as to detect harmful environmental conditions. For example, as mentioned above, gamma-ray radiography was used to locate the rusting iron reinforcements in the Parthenon.
An example of pioneering work in this area is the application of cosmic rays in the search for hidden chambers in the pyramids of Egypt. (11) Cosmic rays were selected as the probe because of their very high energy, which enables them to penetrate many meters of materials. Furthermore, because cosmic rays occur naturally, potentially hazardous manufactured radiation sources are avoided. In 1968, Luis Alvarez, the Nobel prize-winning physicist, placed spark chamber cosmic ray detectors in the burial chamber at the bottom of the Pyramid of Khephren. (See figure 5.) By counting the number of cosmic ray particles reaching the detector from various directions, he was able to compute the amount of material throughout the pyramid. Ultimately, no hidden chambers were detected. This same technique may be applied to scan the interior structure of older bridges. Because of advances in cosmic ray detectors and in portable computers, this scanning will be easier to accomplish than it was in Alvarez's day.
The neutron probe is an example of a nondestructive test for measuring harmful environmental conditions. The probe consists of a portable neutron source and a high-purity germanium detector for gamma rays. This method was used to measure the distribution of chlorides and moisture in historic brick walls at Colonial Williamsburg. (See figure 6.) (12) Its use here was itself a case of technology transfer because it was a spinoff of a method used by the National Aeronautics and Space Administration to map the geology of the moon. Since chlorides from deicing salts or seawater promote the corrosion of the steel in reinforced concrete, the neutron probe could have wide application on highway structures.
Much of the work of architectural conservation involves the repair or strengthening of damaged structures. This damage can be the result of accumulated deterioration of materials or of natural disasters, including floods, fires, earthquakes, or hurricanes. Human activities--ranging from extensive remodeling to military action--can be just as disastrous.
Consequently, conservation scientists have accumulated a vast amount of experience in dealing with extensively damaged structures that, under normal circumstances, would simply be demolished and replaced. (The Leaning Tower of Pisa is an excellent example of this precept.) The specific problems include cracking, loss of section, and settling of foundations.
In these situations, the usual codes for calculating structural stability do not apply; this means that specialized finite-element computer models are needed. For example, a nonlinear model is being developed to predict the structural response of Hagia Sophia to earthquakes. (13) This modeling is especially challenging, partly because of the complexity of the building's design but also because it has already been damaged in previous earthquakes. This type of model will have obvious application to such situations as the Los Angeles highway bridges in the wake of the Northridge earthquake.
Once a structure has been analyzed, it must then be repaired. This often requires innovative approaches to designing internal or external bracing or to underpinning foundations. This work may have to be performed under very confined conditions in historic districts where it is difficult to use conventional cranes or construction equipment. These considerations can also arise in the renewal of highway structures in built-up urban areas.
Unfortunate experiences in architectural conservation have demonstrated the importance of understanding the interactions between repair materials and the original structural materials. Misguided restoration efforts have caused more damage than they have cured. On occasion, thin cementitious grout has been injected into the walls of historic buildings in order to consolidate them. One result has been the formation of soluble salts such as magnesium carbonates and sulfates that form damaging surface efflorescences. In cases where the original material contained significant amounts of gypsum, chemical reactions can lead to disastrous expansive forces. This process caused such severe tensile stresses in the brick walls of the Marienkirche in Lubeck, Germany, the iron reinforcements snapped, and the walls had to be extensively rebuilt. (14)
Another recurring problem in architectural conservation is the application of various kinds of plastic coatings to stone and other porous materials to consolidate or waterproof them. Aside from aesthetic problems, these coatings typically produce surface layers with different physical properties than the underlying material. These differences can, under thermal cycling, lead to significant interfacial stresses, which can, in turn, lead to spalling of the surface. The creation of an impermeable barrier can trap moisture and salts at the interface, again leading to failure. This latter problem is illustrated by the photograph of a Venetian stone sculpture.
Conservation science has much to offer to the field of highway research and development. This type of technology transfer becomes especially important with the completion of the Interstate Highway System. In the future, the emphasis will no longer be on new construction but rather on the conservation of the transportation infrastructure already in place.
The Conservation of Historic Roads and Bridges
Highways, themselves, can be historic architecture in need of conservation. To qualify for listing on the National Register of Historic Landmarks, a structure needs to be at least 50 years old. The average age of highway bridges in the New York City area is 67 years. Construction of the interstate system began in the 1950s; in the next decade, stretches of it could be registered as historic landmarks.
Besides being old, a historic landmark must be significant to the cultural heritage of the United States. A prime example of a historically significant highway structure is the Brooklyn Bridge, which was built over 100 years ago and still serves as a critical part of the highway system. The construction of this bridge united Manhattan to Long Island, with enormous consequences for the urban development of New York City. It also established the suspension method as the preferred way of spanning long distances and greatly enhanced the worldwide reputation of American civil engineering. For all these reasons, the Brooklyn Bridge has been registered as a national historic landmark.
Some historic bridges have more in common, in terms of their construction materials, with medieval cathedrals than with modern steel or reinforced concrete structures. Although the Brooklyn Bridge pioneered the use of steel in large engineering structures, its steel suspension cables are linked to the anchorages by wrought iron chains. (15) Wrought iron has been used since antiquity to strengthen and tie together large stone structures. Moreover, the towers of the Brooklyn Bridge are massive load-bearing granite masonry, built by masons stone by stone--just as in the medieval cathedrals. John Roebling, the engineer of the Brooklyn Bridge, emphasized this resemblance by including Gothic arches in the architectural design of the towers. Consequently, lessons learned in the conservation of Gothic cathedrals can be directly relevant to renewal of these highway bridges.
Many historic bridges have sculptural features that must be conserved. For example, the Taft Bridge in Washington, D.C., is now undergoing restoration. The precast concrete lion sculptures at either end of the bridge require protection from the weather, and they are being replaced with bronze replicas.
Even stretches of highways may be preserved. Under the historic highways program, parts of U.S. Route 66 are being protected as an important part of the American culture.
(1) Strategic Issues in Civil Infrastructure Systems Research: Executive Summary, National Science Foundation, Engineering Directorate, Strategic Planning Committee, Arlington, Va., 1993.
(2) R.A. Livingston. "Architectural Conservation and Applied Mineralogy," Canadian Mineralogist, Vol. 24, 1986, pp. 307-22.
(3) R.A. Livingston. "Influence of the Environment on the Patina of the Statue of Liberty," Environmental Science and Technology, Vol. 25, No. 8, 1991, pp. 1400-08.
(4) R.G. Newton. "The Weathering of Medieval Stained Glass," Journal of Glass Studies, Vol. 7, 1975, pp. 161-68.
(5) R. Helmuth, D. Stark, S. Diamond, and M. Monranville-Regourd. Alkali-Silica Reactivity: An Overview of Research, SHRP-C-342, National Research Council, Strategic Highway Research Program, Washington D.C., 1993.
(6) R.A. Livingston, P.E. Stutzman, R. Mark, and M. Erdik. "Preliminary Analysis of the Masonry of the Hagia Sophia Basilica, Istanbul," Materials Issues in Art and Archaeology III: Materials Research Society Symposium 267, ed. P.B. Vandiver, et al., Materials Research Society, Pittsburgh, Pa., 1992, pp. 721-36.
(7) D.L. Rayment and K. Pettifer. "Examination of Durable Mortar from Hadrian's Wall," Materials Science and Technology, 1987, pp. 997-1004.
(8) J. Mishara. "Early Hydraulic Cements," Early Pyrotechnology, ed. T. Wertime and S. Wertime, Smithsonian Institution Press, Washington, D.C., 1982, pp. 125-34.
(9) M. Suheyl Akman. "Research Activities on Pozzolans in Turkey," Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete: Proceedings of the U.S.-Turkey Workshop, ed. V. Ramakrishnan and M. Suheyl Akman, South Dakota School of Mines & Technology, Rapid City, S.D., 1992, pp. 1-6.
(10) J. Connan and O. Deschesne. "Archaeological Bitumen: Identification, Origins and Uses of an Ancient Near Eastern Material," Materials Issues in Art and Archaeology III: Materials Research Society Symposium 267, ed. P.B. Vandiver, et al., Materials Research Society, Pittsburgh, Pa., 1992, pp. 683-720.
(11) L. Alvarez, et al. "Search for Hidden Chambers in the Pyramids," Science, Vol. 167, 1970, pp. 832-39.
(12) R.A. Livingston and T.H. Taylor, Jr. "Diagnosis of Salt Damage at a Smokehouse in Colonial Williamsburg," Bulletin of the Association for Preservation Technology, Vol. XXIII, No. 3, 1992, pp. 3-12.
(13) A.S. Cakmak, R. Davidson, C.L. Mullen, and M. Erdik. "Dynamic Analysis and Earthquake Response of Hagia Sophia," Structural Repair and Maintenance of Historical Buildings III, ed. C.A. Brebbia and R.J.B. Frewer, Computational Mechanics Publications, Southampton, U.K., 1993, pp. 67-84.
(14) R.A. Livingston, A. Wolde-Tinsae, and A. Chaturbahai. "The Use of Gypsum Mortar in Historic Buildings," Structural Repair and Maintenance of Historic Buildings, II, ed. C.A. Brebbia, J. Dominguez, and F. Escrig, Computational Mechanics Publications, Southampton, U.K., 1991, pp. 157-65.
(15) David McCollough, The Great Bridge, Simon and Schuster, N.Y., 1972.
Richard A. Livingston is the acting chief of the Physical Research Division of the Office of Advanced Research at the Turner-Fairbank Highway Research Center in McLean, Va. He received his bachelor's degree in history from Dartmouth College (1968), a master's degree in nuclear engineering from Stanford University (1970), and a doctorate in geology from the University of Maryland (1990). His professional interests concern the materials science and nondestructive testing of construction materials. During his career, he has worked in research at the Atomic Energy Commission, Environmental Protection Agency, and National Institute of Standards and Technology. Dr. Livingston has also served as a consultant for the conservation of several architectural monuments, including the Statue of Liberty, the Washington National Cathedral, Colonial Williamsburg, Westminster Abbey, the Taj Mahal, and Hagia Sophia.